It is known that a single thyristor may be fired rapidly and safely and with a moderate demand for control power by first supplying the thyristor with an initial control pulse with high amplitude and steepness upon firing, and thereafter supplying the thyristor with a continuous control current with lower amplitude. The initial pulse results in rapid and safe firing and the subsequent lower-amplitude control current maintains the thyristor in the conducting state with a moderate demand for control power. The technique may also be used for simultaneous firing of a plurality of, for example, series-connected thyristors. In this case, the high and steep initial pulse ensures a rapid, reliable and simultaneous firing of the thyristors independent of the variation of the firing properties of the thyristors and of inevitable differences in the parameters of the firing circuits of the individual thyristors.
In equipment of the above-mentioned type, a number of control pulse transformers, typically one for each thyristor, are arranged with their primary windings series-connected and connected to a control current source. The secondary windings of the transformers are connected through rectification means to control electrodes of the thyristors. The control current source may comprise a capacitor which is charged to a relatively high voltage, and which is connected, typically through a transistor switch means, to the series-connected primary windings to provide the required initial pulse. After the end of the initial pulse, the transistor switch means, is pulsed with relatively high frequency so that a pulsed direct current from a current source with lower amplitude is fed through the primary windings of the control pulse transformers. After being transformed to the secondary windings, this current after rectification is supplied to the control electrodes of the thyristors and constitutes the continuous control current which flows after the initial pulse.
However, as the number of series-connected thyristors becomes greater, the required voltage for the initial pulse becomes so high that a transistor is not suitable for use as a switching element. While the use of a thyristor for this element is theoretically possible, as a practical matter it is unsuitable because among other things, the use of a thyristor would require special turn-off circuits to effect the necessary pulsing of the primary current after the end of the initial pulse.
German Auslegeschrift No. 1638378 discloses an arrangement in which each of a plurality of series-connected thyristors is provided with two control pulse transformers, one of which provides the short and steep initial pulse and the other of which provides the subsequent continuous control current. In one embodiment of this arrangement, the thyristors are provided with separate control current sources and separate switching members for triggering the control pulses. This type of circuit is complicated and expensive and does not guarantee simultaneous firing of the individual thyristors. In another embodiment, the primary windings of the transformers which transmit the initial pulse are connected in series across a first voltage source and a first switching element, and the primary windings of the transformers which transmit the continuous control current are connected in series across a second voltage source and a second switching element. However, if a large number of series-connected thyristors are used, both of the switching elements must be thyristors because of the high voltage required, which then results in the above-mentioned disadvantages. Further, if the control pulse transformers are designed in a known manner as high voltage cables having cores fitted on them, two cables are required, one for the initial pulse and one for the continuous control current, which is expensive as well as requiring an excessive amount of space.
The Swedish published patent application No. 363,940 discloses a firing circuit in which the parallel combination of a thyristor and a transistor is connected in series with a voltage source and the primary winding of a control pulse transformer. The thyristor is first fired and triggers the initial pulse, whereupon the transistor is switched into the conducting state and carrys the continuous control current. However, since the transistor, which cannot handle voltages of high magnitude, is subjected to the same voltage as the thyristor, this arrangement can only be used for a relatively small number of series-connected thyristors.